Problem A. 558. (March 2012)
A. 558. Prove that there exists a constant C>0 for which the following statement holds: if n is a positive integer and are sets such that every two of them has at least two, and every three of them has at most three elements in common, then N<Cn2.
(Proposed by: Z. Gyenes, Budapest)
Deadline expired on 10 April 2012.